The field of the invention is nuclear magnetic resonance imaging methods and systems.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0) , the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When applied to cardiac imaging, for example, a complete scan from which a series of images showing the heart at different phases of its cycle can be acquired in a single breath-hold.
The prognosis of patients with a wide variety of cardiac diseases (including coronary artery disease, valvular heart disease, congestive heart failure and cardiac arrhythmias) has been closely linked to the degree of left ventricular ejection fraction a quantitative measure of global ventricular function. Quantitative measures of regional contractile function could have significant prognostic and therapeutic importance. For example, many patients with severe coronary artery disease may have normal regional and global left ventricular function at rest but have abnormalities induced by stress. In clinical practice, patients with coronary artery disease can be detected by stress echocardiography based on new functional deficits during stress. However, interobserver variability of this type of qualitative measure is an inherent limitation that could be improved with quantitative measures. Thus, there is a need for high quality quantitative measures of regional cardiac function.
Invasive measurements of regional contractile function based on ventriculography are limited by arrhythmias induced by the injection, the failure to visualize the myocardium directly, and the consequent limited ability to differentiate endocardial motion due to regional contraction from endocardial motion due solely to tethering. The latter arguments also apply to noninvasive ventriculographic measures of contractile function such as radionuclide ventriculography and echocardiographic analysis of endocardial motion. Regional wall thickening by echocardiography, CT, or MRI is a better measure of regional function but image resolution and irregular myocardial contours limit quantitation. These methods are also susceptible to through plane motion and thus do not image the same myocardium throughout the cardiac cycle. Echocardiography windows and poor endocardial definition, particularly after surgery, cause further compromise. Regional wall thickening using gated SPECT Tc-sestamibi has the appeal of both perfusion and contraction in the same exam. It remains to be seen, however, whether the relatively low image resolution and further degradation due to respiratory motion can be overcome in this prior method to produce quantifiable measures of regional contractile function that have clinical use.
MRI has been used to quantify regional myocardial function using two distinct methodologies: tagging; and velocity encoded phase contrast. Both methods can noninvasively assess specific parts of the myocardium and are inherently quantifiable. While myocardial tagging techniques such as that proposed by Axel L. Dougherty L: "MR Imaging Of Motion With Spatial Modulation Of Magnetization," Radiology 1989; 171:841-845; and Zerhouni E A, Parish D M, Rogers W J, Yang A, Shapiro E P: Human Heart: "Tagging With MR Imaging--A Method for Noninvasive Assessment of Myocardial Motion," Radiology 1988; 169:59-63, have received considerable attention in recent years, the tag spacing must be several times larger than the image resolution to allow accurate localization. Within the limits set by tag spacing, the deformations of the tags theoretically can be quantified with a spatial accuracy significantly better than the image resolution. Finite element analysis of myocardial tags has been used to quantify regional myocardial mechanics in a variety of heart diseases.
Velocity encoded phase contrast MRI can be used as described by Pelc N J, Drangova M, Pelc L R, Zhu Y, Noll D C, Bowman B S, Herfkens R J: "Tracking Of Cyclic Motion With Phase-Contrast Cine MR Velocity Data," J. Magn Reson Imaging 1995; 5:339-345; Pelc L R, Sayre J, Yun K, Castro L J, Herfkens R J, Miller D C, Pelc N J: "Evaluation Of Myocardial Motion Tracking With Cine-Phase Contrast Magnetic, Resonance Imaging," Invest Radiol 1994; 29:1038-1042; and Wedeen V J: "Magnetic Resonance Imaging of Myocardial Kinematics, Technique to Detect, Localize, and Quantify The Strain Rates Of The Active Human Myocardium," Magn Reson Med 1992; 27:52-67, to track the position of a voxel of myocardium across the cardiac cycle based on its velocity and acceleration as a function of time. To date however, motion tracking methods are sensitive to accumulated errors that can result in position errors of 1-2 pixels. Alternatively, phase contrast velocity data can be analyzed in strict mechanical terms as the strain rate. Strain rate analysis uses the three or four nearest neighbor velocity determinations to correct for local translation and rotation on a pixel-by-pixel basis with final estimation of the major and minor axes of regional deformation. Since normal myocardial deformation is primarily radially oriented, the major axis of strain rate is roughly equivalent to the time derivative of wall thickening/thinning. Strain rate analysis as implemented thus far requires multiple differential calculations and thus is susceptible to noise. It also is sensitive to partial volume problems since it requires information from surrounding pixels.
The basic premise of all these MRI methods has been that gross motion of the heart through the chest should be eliminated to reveal the deformation associated with heart contractile function. However, the MRI system measures the spin motions relative to the stationary reference of the MRI system. As a result, velocities acquired from the myocardium are a combination of 3-dimensional rotation, translation, and deformation of the heart coordinated with the cardiac cycle. Respiratory motion contributes additional rotations and translations through the chest. Abnormal contractile patterns such as bundle branch block further complicate the interpretation of myocardial velocities from the external stationary frame of reference. Thus, a number of parameters modify the velocity of a region of the heart relative to an external reference point on the MRI system.